The present invention relates to a mass spectrometer and method of mass spectrometry.
Curved or non linear geometry RF ion guides are known. Curved geometry ion guides allow more compact mass spectrometers to be designed compared to mass spectrometers with linear ion guides. Non linear geometry ion guides may also be used to reduce the amount of neutral or non-ionised species reaching an ion detector.
In some commercial mass spectrometers a gas filled curved geometry RF ion guide may be utilised as a collision gas cell. The pressure of the gas (e.g. Argon) within the collision gas cell is generally between 10−3 to 10−2 mbar.
Parent or precursor ions which are accelerated into the collision cell are fragmented by Collisionally Induced Dissociation (“CID”) to form product ions. The product ions are then analysed by a downstream mass analyser. In some cases parent or precursor ions may be selected by an upstream mass filter prior to fragmentation.
In a conventional RF ion guide radial confinement is achieved by applying inhomogeneous fields oscillating at RF frequencies. Application of these oscillating fields results in a pseudo-potential which acts to confine ions within the ion guide.
The pseudo-potential (R,Z) within an RF ring stack comprising a plurality of electrodes each having an aperture as a function of radial distance R and axial position Z is given by:
                              Ψ          ⁡                      (                          R              ,              Z                        )                          :=                                            z              ·              e              ·                              Vo                2                                                    4              ·              m              ·                              ω                2                            ·                              Zo                2                                              ·                                                    I                ⁢                                                                  ⁢                1                ⁢                                                                            (                                              R                        Zo                                            )                                        2                                    ·                                                            cos                      ⁡                                              (                                                  Z                          Zo                                                )                                                              2                                                              +                              I                ⁢                                                                  ⁢                0                ⁢                                                                            (                                              R                        Zo                                            )                                        2                                    ·                                                            sin                      ⁡                                              (                                                  Z                          Zo                                                )                                                              2                                                                                      I              ⁢                                                          ⁢              0              ⁢                                                (                                      Ro                    Zo                                    )                                2                                                                        (        1        )            wherein m is the mass of the ion, e is the electronic charge, Vo is the peak RF voltage, ω is the angular frequency of the RF voltage, Ro is the radius of the aperture. Zo.π is the centre to centre spacing between ring electrodes, I0 is a zeroth order modified Bessel function of the first kind, and I1 is a first order modified Bessel function of the first kind.
The RF voltage applied to adjacent ring electrodes is preferably 180° out of phase.
The pseudo-potential field for a quadrupole rod set ion guide as a function of radial distance r is given by:
                                          V            *                    ⁡                      (            r            )                          =                              e            ·                          V              0              2                        ·                          r              2                                            4            ⁢                                                  ⁢            ω            ⁢                                                  ⁢                          mr              0              4                                                          (        2        )            wherein r0 is the internal radius of the quadrupole rod set.
The RF voltage applied to one set of opposing rods is 180° out of phase to that applied to the other set of opposing rods.
From Eqns. 1 and 2 it can be seen that the amplitude of the pseudo-potential is inversely proportional to the mass to charge ratio of ions within the guide.
In order to perform CID fragmentation, parent or precursor ions are arranged to enter the collision gas cell from a region maintained at a relatively low pressure with a kinetic energy which is sufficient to cause fragmentation of the parent or precursor ions by collisions with the target gas. The ions may be arranged to have a kinetic energy of between 10 and 100 eV. Ions entering the gas cell lose kinetic energy as they collide with the target gas and eventually reach thermal energy. This process is called collisional cooling.
However, at the entrance of a curved gas cell where ions have highest kinetic energy, the pseudo-potential field acts in the opposing direction to the direction in which the ions are travelling and must be sufficiently high to ensure that ions are effectively confined within the gas cell during the period in which collisional cooling is occurring. If the confining force is too small then ions may be lost by collision with the electrodes or may exit the ion guide in a radial direction.
As the pseudo-potential force is inversely dependent on the mass to charge ratio of ions, the amplitude of the RF potential must be increased for higher mass to charge ratio ions to minimise these losses. At higher RF amplitudes low mass to charge ratio product ions from high mass to charge ratio parent or precursor ions may be lost due to mass instability within the RF field. This low mass cut-off effect is well known in RF devices operated at high voltage.
U.S. Pat. No. 6,891,157 discloses a curved ion guide.
WO 2005/067000 discloses an ion extraction device.
WO 2009/036569 discloses a collision cell having a curved section.
It is desired to provide an improved device.